1. Field of the Invention
Embodiments of the present invention generally relate to the deposition of thin films using chemical vapor deposition processing. More particularly, this invention relates to a process for depositing thin films onto large area substrates.
2. Description of the Related Art
Organic light emitting diode (OLED) displays have gained significant interest recently in display applications in view of their faster response times, larger viewing angles, higher contrast, lighter weight, lower power and amenability to flexible substrates, as compared to liquid crystal displays (LCD). Practical application of OLED is enabled by, as shown in FIG. 1, using layers of organic materials 103 sandwiched between two electrodes 102, 104 and built on a substrate 101. For example, two organic layers, in contrast to the old single organic layer, including one layer capable of monopolar (hole) transport and the other layer for electroluminescence can be used together with an anode layer and a cathode layer to lower the required operating voltage for OLED display. The cathode layer generally includes a metallic material and the anode layer may include a transparent material, such as an indium tin oxide (ITO) material, to be disposed on the bottom next to the substrate or on top of the OLED device for emitting light in a top emission device or a bottom emission device, respectively. Organic thin film transistor (TFT) device, active matrix devices, and other devices may also include additional structure, such as a TFT structure.
Over the years, layers in display devices have evolved into multiple layers with each layer serving different function. For example, a stack of organic layers could comprise a hole-injection layer, a hole-transport layer, an emissive layer, an electron-transport layer, and an electron injection layer. It should be noted that not all the above five layers of organic layers are needed to build an OLED cell. For example, in some cases, only a hole-transport layer and an emissive layer are needed. When an appropriate voltage (typically a few volts) is applied to the cell, the injected positive and negative charges recombine in the emissive layer to produce light (electroluminescence). The structure of the organic layers and the choice of the anode and cathode layers are designed to maximize the recombination process in the emissive layer, thus maximizing the light output from the OLED devices.
Many polymer materials, in addition to organic materials used in OLED, are also developed for small molecule, flexible organic light emitting diode (FOLED) and polymer light emitting diode (PLED) displays. Many of these organic and polymer materials are flexible for the fabrication of complex, multi-layer devices on a range of substrates, making them ideal for various transparent multi-color display applications, such as thin flat panel display (FPD), flexible displays, electrically pumped organic laser, and organic optical amplifier. The lifetime of display devices can be limited, characterized by a decrease in EL efficiency and an increase in drive voltage, due to the degradation of organic or polymer materials, the formation of non-emissive dark spots, and crystallization of the organic layers at high temperature of about 55° C. or higher, e.g., crystallization of the hole transport materials. Therefore, a low temperature deposition process for these materials, such as at about 100° C. or lower is needed.
In addition, the main reason for the material degradation and dark spot problems is moisture and oxygen ingress. For example, exposure to humid atmospheres is found to induce the formation of crystalline structures of 8-hydroxyquinoline aluminum (Alq3), which is often used as the emissive layer, resulting in cathode delamination, and hence, creating non-emissive dark spots growing larger in time. In addition, exposure to air or oxygen may cause cathode oxidation and once organic material reacts with water or oxygen, the organic material is dead. Currently, most display manufacturers use metal-can or glass-can materials as an encapsulation layer to protect organic materials in the device from water (H2O) or oxygen (O2) attack. The metal or glass materials are attached to the substrate like a lid using a bead of UV-curved epoxy. However, moisture can easily penetrate through the epoxy and damage the device.
Other materials, such as inorganic materials, e.g., silicon nitride (SiN), silicon oxynitride (SiON) and silicon oxide (SiO), prepared by plasma enhanced chemical vapor deposition (PECVD), can also be used as an effective encapsulation/barrier layer against moisture, air and corrosive ions for such devices. For example, as shown in FIG. 1, an encapsulation/barrier layer 105 is deposited on top of the substrate 101 to protect the electrodes 102, 104 and the organic materials 103 underneath. However, it is very difficult to generate water-barrier inorganic encapsulation materials using a low temperature deposition process because the resulting film is less dense and has high defect pinhole structures. It is important to note that the presence of residual moisture in the organic layers may also promote the Alq3 crystallization process even in encapsulated devices. In addition, oxygen and humidity being trapped during encapsulation and infiltrating into the OLED device to be in contact with the cathode and organic materials generally result in dark spot formation, which is a frequent OLED destroying defect. Therefore, a good encapsulation/barrier film also requires low water vapor transmission rate (WVTR).
Other problems also arise when thin film inorganic silicon nitride (SiN) related materials are used as the encapsulation/barrier layer. When the encapsulating layer is deposited to be thick to serve as a good oxygen and water barrier, it is usually hard, fragile, and too thick to adhere well to a substrate surface, resulting in cracking or peeling off the substrate surface, especially at high temperature and humidity stressed conditions. If the encapsulating layer is made thin to improve adhesion and thermal stability, it is not thick enough as a moisture barrier. Thus, additional stress relieving layers or other manipulation may be required. For example, we have previously used one or more low dielectric constant material layers alternating with one or more inorganic encapsulation/barrier layers to improve water-barrier and thermal stress performance and protect the devices underneath. However, despite having good water-barrier performance, the low dielectric constant materials are not very transparent and light transmittance is worse when multiple layers are used. As such, alternating layers of low dielectric constant materials are not compatible for some applications that require light emitting through the encapsulation multilayer. FIG. 2 demonstrates a direct correlation between light transmitted in a visible light spectrum and different thicknesses of low dielectric constant material films. With lines 240 representing the thickest film and line 210 representing the thinnest film, the transmittance of the four dielectric constant material films are especially poor at lower wavelengths for lines 210, 220, 230, 240, which will directly affect the quality of light emission at these wavelengths, leading to bad color display.
Thus, there is still a need for methods of depositing multiple layers of stress relieving materials and encapsulation/barrier materials with improved transmittance onto large area substrate.